The present invention relates to a method for improved antilock braking control for all-wheel drive vehicles having a viscous coupling or a viscous lock.
Electronic antilock braking systems (xe2x80x9cABSxe2x80x9d) have been known and used for several years. An example of an electronic antilock braking system as implemented in a vehicle dynamic control system is disclosed in European Patent Application EP 0 503 025 B1. That braking system operates based on calculated wheel torques or wheel braking pressures, but does not account for differences in wheel torques or braking pressures due to the viscous torque from a viscous coupling or a viscous lock in an all-wheel drive vehicle.
Some all-wheel drive vehicles use a viscous coupling (an encapsulated multi-plate unit with a high-viscosity silicone fluid) as a means of activating the all-wheel drive. Once the traction limit at the permanently-engaged axle is exceeded, the viscous coupling responds to variations in slip by transferring torque to the secondary drive axle according a viscous-drive response curve. In a vehicle with a viscous lock, because of the presence of a center differential, the driving torque is permanently distributed to the front and rear axles (with the ratio depending on the design, for example ⅓ to the front axle and ⅔ to the rear axle). The viscous lock, like the viscous coupling, can transfer supplemental torque between the front and rear axle when there is a speed difference between the front and rear axle.
Because known ABS control systems do not adequately account for the variations in torque at each wheel due to the transfer of torque by the viscous coupling or viscous lock, these systems often apply the wrong braking pressure to the wheels resulting in less than optimal braking performance.
The present invention relates to a method for improved antilock braking control for all-wheel drive vehicles having a viscous coupling or a viscous lock by taking into account the torque due to the viscous coupling or viscous lock which is acting on a wheel. The present invention also relates to the improvement of the formation of reference values for the torque acting upon a wheel of an all-wheel drive vehicle having a viscous coupling or viscous lock.
By more accurately taking into account of the torque applied by the viscous coupling or viscous lock on each wheel, an ABS control unit can more accurately respond by increasing or decreasing the braking pressure on the wheels as necessary, resulting in improved braking performance.
Specifically, the present invention relates to a method for controlling a braking system in an all-wheel drive vehicle having a viscous coupling or a viscous lock, wherein a viscous torque acting on a wheel is estimated, the viscous torque is taken into account in estimating a desired braking pressure for the wheel, and the desired braking pressure is applied to the wheel.
If the viscous torque acting on the wheel is not known to a precise degree, it may be advantageous to take the viscous torque into account in calculating the desired braking pressure only if the viscous torque is less than or equal to zero. The viscous torque is calculated as a function of a difference between a front axle cardan shaft speed and a rear axle cardan shaft speed, and can also be taken into account in estimating a braking force at the wheel. A reduction factor can be introduced to adjust the effect of the viscous torque for high differences in front and rear axle cardan drive shaft speeds. For example, the reduction factor can be equal to approximately one for small differences in front and rear axle cardan shaft speeds and can be equal to approximately zero for large differences. Small differences in speed is an indication that the viscous coupling or viscous lock is functioning normally and large differences in speed is an indication that the viscous coupling or viscous lock is functioning poorly.